1. Field of the Invention
The present invention relates to a surface inspection apparatus and a surface inspection method for a strained silicon wafer to inspect a surface strain caused by a dislocation (misfit dislocation) that occurs in a process of manufacturing the strained silicon wafer.
2. Description of the Related Art
In recent years, attention has been given to a strained silicon wafer to increase the operation speed of a silicon semiconductor device. For example, as shown in FIG. 11A, a strained silicon wafer 10 has a structure, in which a silicon substrate layer (Si substrate layer) 11 formed of monocrystalline silicon and a silicon germanium layer (SiGe layer) 12 having lattice spacing greater than that of the monocrystalline silicon and crystal-grown on the Si substrate layer 11, and a silicon layer (Si layer) 13 crystal-grown on the SiGe layer 12. The SiGe layer 12 is formed of a composition gradient SiGe region 12a, in which the germanium (Ge) concentration is gradually increased in the direction of thickness, and a relaxed SiGe region 12b, in which the germanium (Ge) concentration is substantially constant and which is formed on the composition gradient SiGe region 12a. 
The strained silicon wafer 10 is not limited to the structure described above, but may have structures as shown in FIGS. 11B, 11C and 11D. In the example shown in FIG. 11B, an SiGe layer 12 of a uniform composition is formed on an Si substrate layer 11, and an Si layer 13 is crystal-grown on the SiGe layer 12. In the example shown in FIG. 11C, a silicon oxide layer 14 is formed on an Si substrate layer 11, a relaxed SiGe layer 12b is crystal-grown on the silicon oxide layer 14, and an Si layer 13 is crystal-grown on the relaxed SiGe layer 12b. This is called SGOI (Silicon Germanium on Insulator). In the example shown in FIG. 11D, a silicon oxide layer 14 is formed on an Si substrate 11, and an Si layer 13 is crystal-grown on the silicon oxide layer 14. This is called SSOI (Strained Silicon on Insulator).
In the strained silicon wafer 10 of the structure described above (refer to FIG. 11A), since the Si layer 13 is crystal-grown (epitaxial-grown) on the SiGe layer 12 (relaxed SiGe region 12b) having relatively great lattice spacing, strain occurs in the Si layer 13 (this Si layer is hereinafter referred to as the strained Si layer). Strain also occurs in the SiGe layer 12 because of lattice misfit between the Si substrate layer 11 and the SiGe layer 12 formed thereon. Because of these strains, specifically, dislocation (misfit dislocation) continuously occurs in the strained Si layer 13 in a direction along the lattice structure.
To judge whether the manufacturing process is appropriate or not, it is helpful to know a state of the strain caused by the dislocation in the strained Si layer 13 as a surface layer of the strained silicon wafer 10. The state of the strain caused by the dislocation in the strained Si layer 13 can be observed by X ray topography (XRT) (see, for example, “Analysis Handbook for ULSI Manufacturing” edited by Tsuneo Ajioka and Michihiko Inaba, pp. 392-397, Realize Inc., 1994). The X ray topography (XRT) is a method for observing a spatial distribution or size of crystal defects or lattice strain, utilizing diffraction of an X ray. More specifically, a diffraction ray only from a specific lattice surface is taken out. Then, a subtle contrast in the diffraction image, which occurs due to some defect resulting from, for example, dislocation in the diffraction image is observed in one-to-one correspondence with each part of the sample crystal. However, the inspection using the X ray topography (XRT) is basically destructive testing; that is, since the X ray need be irradiated on the strained silicon wafer 10 as a sample, the inspected strained silicon wafer cannot be utilized thereafter. In addition, since the X ray is used, it is troublesome to control the testing area.
When dislocation occurs in the process of growing a silicon layer on the SiGe layer 12 (relaxed SiGe region 12b), asperity strain resulting from the dislocation is generated on the surface of the strained Si layer 13, which is formed upon completion of the growth. Since the asperity strain on the surface is caused by the dislocation extending in the direction of the lattice structure of the strained Si layer 13, it may exhibit a lattice-shaped pattern (hereinafter referred to as cross-hatch pattern).